Internal combustion engines are commonly used in today's automobiles. Since inducted air is used to burn fuel and produce power in this type of engine, engine power may be limited by the amount of air that can be inducted into the combustion chamber. Turbochargers may be used to increase the air inducted to the combustion chamber compared with a naturally aspirated system. Further, some automobiles may employ a dual turbocharger system, which can reduce turbo lag while maintaining peak boosting performance by allowing the operation of a single turbocharger at lower engine speed and the operation of dual turbochargers at higher engine speed.
One type of dual turbocharger system is described in U.S. Pat. No. 5,186,005. In this particular dual turbocharger system, twin turbochargers are arranged in a parallel fashion with respect to the engine, and a crossover pipe connects the twin turbochargers. One turbocharger is operated at low engine load/speed while both turbochargers are operated at high engine load/speed. The switch from the single turbocharger operation to the dual turbocharger operation is based on the intake air quantity. However the switch back to the single turbocharger operation is based on the speed, that is, once a dual turbocharger operation is achieved it is only changed back to a single turbocharger operation when the engine speed is below a certain set value, irrespective of the intake air quantity change.
However, one issue associated with such dual turbocharger systems and possibly other dual turbocharger systems is that it may be difficult to maintain the exhaust temperature within a proper range for the optimal operation of a device in the emission control system for reducing emissions in the exhaust. For example, if the exhaust temperature is too cold, the efficiency of the device may be low; and if the exhaust temperature is too hot, the device may degrade physically or chemically in the example of a catalytic device. During some conditions, for example when the engine has just been started or when the engine is operated at a low speed, the exhaust temperature may be too low for efficient operation or catalytic conversion. Yet during other conditions, for example when the engine is operated at a high speed or load, the exhaust temperature may be too high, which may cause a catalyst to degrade.
The inventors herein have recognized the above issues and that such issues may be at least partially addressed by an exhaust system for an engine having a first set and a second set of cylinders, a first turbocharger coupled to the first set of cylinders and a second turbocharger coupled to the second set of cylinders, and an emission control device. Specifically, the exhaust system may include a lower heat loss path that is coupled to and downstream of the first turbocharger, a higher heat loss path that is coupled to and downstream of the second turbocharger, a crossover pipe coupled between and upstream of the first and the second turbocharger that provides a passage between the higher heat loss path and the lower heat loss path, and a control mechanism for adjusting flow in the crossover pipe.
By providing both a higher heat loss and a lower heat loss path with a crossover that allows communication between the two paths, and by providing a mechanism to control the flow through the higher heat loss and the lower heat loss paths, the exhaust temperature may be better controlled to increase the efficiency of the emission control device operation in a dual turbocharger environment. Under certain conditions, for example, when the engine is operating at a lower load/speed, the exhaust flow through the higher heat loss path is reduced and the exhaust flow through the lower heat loss path is increased to reduce heat loss; while under certain other conditions, for example, when the engine is operating at a higher load/speed, the exhaust flow through the higher heat loss path is increased and the exhaust flow through the lower heat loss path is decreased to increase heat loss from the exhaust.
Further, by placing the crossover between the higher heat loss path and the lower heat loss path upstream of both turbochargers, it is possible to achieve (1) a faster boost response because only one turbocharger has to be spun up and this turbocharger will receive twice as much airflow compared to if both turbochargers have to be spun up, and (2) a faster exhaust warm-up because all exhaust flows through the lower heat loss path and the cooling that may result from air expansion in the second turbocharger is reduced.
As such, it is possible to provide an internal combustion engine that has a dual turbocharger system and an emission control device with mechanisms to control the exhaust temperature for improved operation of the emission control device.
While the above example is illustrated with regard to a dual turbocharger system, the concepts may be equally applicable, if not more applicable, to other turbocharger systems.